Let's remember that the testing cycles in Europe and the US are not the same, and that the 57 MPG number isn't explained (well, I just skimmed... but a search didn't find much).

Is it a combined or just highway rating?

In general, the EU cycle gives numbers 30% higher (and even more if you forget to convert imperial gallons back to US gallons).

So while diesel has benefits, lets be careful with the numbers.